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Andy Douglas' streamliner - Plastazote molding tests
01/17/02: I did some test-molding of various thicknesses and densities of Plastazote, using a big light bulb as a plug. The bulb is 3.7 inches in diameter. I simply heated the foam with a heat gun and forced it around the bulb with my hands.
The foam samples are as follows:
Black: 0.27" HD80
Blue: 0.5" LD45
Large red piece: 0.375" LD45
Small red piece: 0.3" LD60
It was not as difficult to mold the denser foam as I thought it would be, though it behaved rather differently than the less-dense foam. Note that on the less-dense foam my thumbprints are clearly visible.
The LD60 is noticeably more open-grained (rougher) than the LD45. The LD60 behaved quite well while molding.
The HD80 has a very nice semi-gloss finish after molding. It's amazingly rigid. I suspect that this means it will also prove to be much less forgiving to work with than the less-dense foam, making it more difficult to get a good-looking, smooth overall result.
I also tried molding the HD80 around my test plug piece, though the reults are inconclusive (I didn't have time to properly secure the edge of the foam, which meant that the same hand that was forcing the curvature was also holding the foam in place... and staying clear of the heat gun...). This is why the rest of the black piece looks warped. the piece of masking tape is aligned along the nose curve. I did note a lot of springback on the black piece, but again it's hard to tell... a followup experiment is needed.
My initial conclusion is that the critical factor in ease of molding the foam is not density, but thickness. Density certainly does have something to do with it (HD80 at .5" thickness would probably be extremely difficult to work with), but it has a less-pronounced effect than thickness.
The question is, how thin can a given density of foam be before it loses the desired physical properties? Certainly the quarter-inch HD80 is strong enough. But with the color limitations it has (only available in black or white), it's not suitable for the whole fairing.
Perhaps the best compromise is 0.3 or 0.375 LD60. It's more rigid than the LD45, and at the reduced thickness should weigh the same. The tradeoff is that the surface is rougher. If it's only used on the side panels (which are not stressed like the top and bottom are) then reduced strength might not be an issue.
The one guy who has direct experience with all of this stuff is John Tetz. I'll need to talk to him some more as the time approaches to order the foam.